The Master Cylinder
in Action
When you press the brake pedal, it pushes on the primary piston through a
linkage. Pressure builds in the cylinder and lines as the brake pedal is
depressed further. The pressure between the primary and secondary piston
forces the secondary piston to compress the fluid in its circuit. If the brakes
are operating properly, the pressure will be the same in both circuits. If there
is a leak in one of the circuits, that circuit will not be able to
maintain pressure. When the first circuit leaks, the pressure between the
primary and secondary cylinders is lost. This causes the primary cylinder to
contact the secondary cylinder. Now the master cylinder behaves as if it has
only one piston. The second circuit will function normally, but you can see from
the animation that the driver will have to press the pedal further to activate
it. Since only two wheels have pressure, the braking power will be severely
reduced.

The Combination
Valve
You will find a combination valve on most cars with front disc brakes and rear
drum brakes.